The progressive structural and functional development of the blood vessel wall is being studied in embryonic chick blood vessels and in new capillaries induced to proliferate into the corneas of rabbits. The synthesis and deposition of connective tissue components such as glycosaminoglycans, collagen, glycoproteins and elastin, elaborated by endothelial cells, fibroblasts and other perivascular cells is being measured by autoradiography, histochemistry and electron microscopy. The state of these components in the basal lamina, endocapillary layer, media and adventitia is being followed during the growth, maturation, remodelling and regression of the new capillaries. Connective tissue changes will be related to the permeability of the vessel walls to fine structural tracers. Changes in the composition of the corneal stroma that accompany capillary invasion are also being investigated. Studies are also focused on the mechanism of capillary regression.